1. Field of the Invention
The present invention relates to a coil-type slide fastener, wherein a core cord is inserted into coil-shaped fastener elements made of thermoplastic resin, the coil-shaped fastener elements are sewn onto a longitudinal edge of a fastener tape by synthetic fiber yarn, then, the coil-shaped fastener elements are fixed to the fastener tape by thermally melting the sewing yarn of synthetic fiber.
2. Description of the Related Art
A conventional slide fastener of this type is disclosed in Japanese Patent Publication No. 47-29135, partly as shown in FIG. 12. In the Publication, a core cord 3' is inserted into coil-shaped fastener elements 1' made of thermoplastic resin, the fastener elements 1' are sewn onto a longitudinal edge of a fastener tape by multi-thread chain stitch or other sewing means by using sewing yarn of synthetic fiber, then, the sewing yarn is melted in a proper manner so that the sewing threads arranged side by side on the fastener elements 1' are welded to each other to form thick portions, or looper thread is welded to form retaining rings 8' so as to form retaining elements for needle thread 4' of the sewing yarn as shown in FIG. 12, or the looper thread is welded and fixed to shoulders of the fastener elements 1', thereby fixing the fastener elements l' to the fastener tape.
Further, as shown in FIG. 13, there is also an abrasive resistant slide fastener as disclosed in U.S. Pat. No. 5,596,793. According to the U.S. Patent, a core cord 3" is inserted into coil-shaped fastener elements 1" made of thermoplastic resin, the fastener elements 1" are sewn onto a longitudinal edge of a fastener tape by multi-thread chain stitch means by using sewing yarn of synthetic fiber, upper looper portions of looper thread 5" are melted in a state wherein lower looper portions 5a" of the looper thread 5" are drawn toward the interior of the core cord 3" by applying a greater tension to needle thread 4" than to the looper thread 5" during the sewing so as to form anchor portions 8" at end portions of the lower looper portions 5a" on the core cord 3" such that the sewing yarn does not appear on an upper face of the fastener elements 1" fixed to the fastener tape.
In the above-described slide fastener shown in FIG. 12, the sewing yarn appearing on the upper face of the coil-shaped fastener elements 1' are welded to each other to form thick portions, or the looper thread of multi-thread chain stitch are thermally melted to form the retaining rings 8' for the needle thread 4'. Because the synthetic fiber yarn for the looper thread is merely thermally melted, thermal degradation which causes deterioration in quality of material is liable to be generated. As a result, a difference in quality of material is liable to be generated between the thick portions or retaining rings 8' formed by thermal melting and portions which were not thermally melted so that rupture is liable to take place and that the slide fastener may be broken.
In the slide fastener shown in FIG. 13, the greater tension is applied to the needle thread 4" in sewing of the coil-shaped fastener elements 1" on the fastener tape such that entangling points of the needle thread 4" with the looper thread 5" are disposed within the core cord 3" and the upper looper portions of the looper thread 5" are melted to form the anchor portions 8" at the end portions of the lower looper portions 5a". Therefore, when the upper looper portions of the looper thread 5" are melted while the greater tension is being applied to the needle thread 4", the lower looper portions 5a" are drawn deep into the core cord 3" by the needle thread 4" so that restraint strength of the core cord 3" is decreased. As a result, the lower looper portions 5a" may come out through a sewing hole together with the needle thread 4" when an upthrusting operation is applied to the fastener elements 1" in use. Thus, the slide fastener has a problem in its strength.